Coaxial waveguide microstructure having center and outer conductors configured in a rectangular cross-section

ABSTRACT

Provided are coaxial waveguide microstructures. The microstructures include a substrate and a coaxial waveguide disposed above the substrate. The coaxial waveguide includes: a center conductor; an outer conductor including one or more walls, spaced apart from and disposed around the center conductor; one or more dielectric support members for supporting the center conductor in contact with the center conductor and enclosed within the outer conductor; and a core volume between the center conductor and the outer conductor, wherein the core volume is under vacuum or in a gas state. Also provided are methods of forming coaxial waveguide microstructures by a sequential build process and hermetic packages which include a coaxial waveguide microstructure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/085,124 filed Apr. 12, 2011, now U.S. Pat. No. 8,742,874 issued Jun.3, 2014, which is a continuation of application Ser. No. 12/214,785filed Jun. 20, 2008, now U.S. Pat. No. 7,948,335 issued May 24, 2011,which is a continuation application of application Ser. No. 11/637,629filed Dec. 12, 2006, now U.S. Pat. No. 7,405,638 issued Jul. 29, 2008,which is a continuation application of application Ser. No. 11/316,417filed Dec. 22, 2005, now U.S. Pat. No. 7,148,772 issued Dec. 12, 2006,which is continuation application of U.S. Non-Provisional ApplicationNo. 10/793,574 filed Mar. 4, 2004, now U.S. Pat. No. 7,012,489 issuedMar. 14, 2006, which claims the benefit of priority under 35 U.S.C.§119(e) to U.S. Provisional Application Nos. 60/452,073 filed Mar. 4,2003, and 60/474,549 filed May 29, 2003, the entire contents of whichapplications are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to microfabrication technology and toelectronic devices. More particularly, the present invention relates tocoaxial waveguide microstructures and to methods of forming suchmicrostructures, as well as to electronic devices which include suchmicrostructures.

BACKGROUND OF THE INVENTION

Coaxial waveguide microstructures formed by sequential build processeshave been described, for example, in International ApplicationPublication No. WO 00/39854 (WO '854). With reference to FIG. 1A, WO'854 discloses a coaxial waveguide microstructure 100 formed by asequential build process. The microstructure includes an insulatingsubstrate 102, metallic ground lines 104 formed on the substrate 102which are disconnected and divided into two parts, metallic supporters106 formed on the surface of the insulating substrate between thedivided ground lines 104, a signal line 108 on the supporters 106fortransmission of the signals, ground walls 110 formed on the groundlines, and a ground line 112 formed on the ground walls 110. There arevarious disadvantages associated with this type of coaxial waveguidestructure. For example, the use of metal supporters for supporting thesignal line would result in some degree of reflection of the propagatedwaves and thus create signal interference. Further, the process is notreadily scalable to a stacked structure having a plurality of coaxiallayer structures due, for example, to the requirement that the metalsupporters be connected to the dielectric substrate as a means ofsupport. A multi-layer structure is desirable, for example, incrossovers and for realizing a complex distribution network. Inaddition, the choice of a substrate material in this known structure isnot flexible, but is limited to dielectric materials to achieve similarwaveguiding performance. Further, the coaxial waveguide structure cannotbe separated from the substrate due to the requisite mechanicalconnection of the supporters to the substrate.

FIG. 1B illustrates another coaxial waveguide microstructure disclosedin WO '854. The microstructure 114 includes a semiconductor substrate116, first and second ground lines 118, 120, a dielectric supporter 122,space 126 and a signal line 124. The semiconductor has a groove, and thefirst ground line is formed on the surface of the substrate and on thesurface of the groove. In this way, the signal line is formed at almostan identical height as that of the surface of the semiconductorsubstrate, thus allowing easy connection to other connections formed onthe semiconductor substrate. This structure, however, also has variousdisadvantages. For example, the process is not scalable to a multiplecoaxial layer structure due to the requirement of a recessed substrate,the geometry of the waveguide structure, and the requirement for agroove on a planar substrate to achieve the requisite planarization.Finally it is not clear that the structure could be self-supporting andthus removed from the substrate, which would be useful, for example, ina stacked coaxial network.

There is thus a need for improved methods of forming coaxial waveguidemicrostructures which overcome or conspicuously ameliorate one or moreof the problems described above with respect to the related art.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, provided are coaxialwaveguide microstructures. The microstructures include a substrate and acoaxial waveguide disposed above the substrate. The coaxial waveguideincludes: a center conductor; an outer conductor including one or morewalls, spaced apart from and disposed around the center conductor; oneor more dielectric support members for supporting the center conductorin contact with the center conductor and enclosed within the outerconductor; and a core volume between the center conductor and the outerconductor, wherein the core volume is under vacuum or in a gas state.

In accordance with a further aspect of the invention, provided aremethods of forming coaxial waveguide microstructures by a sequentialbuild process. The methods involve: (a) depositing a plurality of layersover a substrate, wherein the layers include one or more of a metalmaterial, a sacrificial photoresist material, and a dielectric material,thereby forming a structure above the substrate, which includes: acenter conductor; an outer conductor having one or more walls, spacedapart from and disposed around the center conductor; one or moredielectric support members for supporting the center conductor, incontact with the center conductor and enclosed within the outerconductor; and a core volume between the center conductor and the outerconductor, wherein the core volume includes sacrificial photoresist; and(b) removing the sacrificial photoresist from the core volume.

Other features and advantages of the present invention will becomeapparent to one skilled in the art upon review of the followingdescription, claims, and drawings appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed with reference to the followingdrawings, in which like reference numerals denote like features, and inwhich:

FIGS. 1A-1B illustrate cross-sectional views of known coaxial waveguidemicro structures;

FIGS. 2-13 illustrate cross-sectional and top-down views of a firstexemplary coaxial waveguide microstructure during various stages offormation in accordance with the invention;

FIGS. 14-26 illustrate cross-sectional and top-down views of a secondexemplary coaxial waveguide microstructure during various stages offormation in accordance with the invention;

FIGS. 27 and 28, respectively, illustrate cross-sectional views of aplurality of the first and second exemplary coaxial waveguidemicrostructures in a stacked arrangement in accordance with theinvention;

FIGS. 29-31 illustrate cross sectional views of exemplary coaxialwaveguide microstructures in a stacked arrangement and connected byvias, in accordance with the invention;

FIG. 32 illustrates two waveguides in the form of a signal or powercoupler, in accordance with the invention;

FIGS. 33A-33B illustrate exemplary structures for transitioning betweencoaxial and non-coaxial waveguide microstructures, in accordance withthe invention;

FIG. 34 illustrates an exemplary waveguide microstructure affixed to asubstrate by a flexible structure, in accordance with the invention.

FIG. 35 illustrates an exemplary capped structure for preventingcontamination of the coaxial waveguide microstructures, in accordancewith the invention;

FIGS. 36-38 illustrate various exemplary structures for interconnectingwaveguide microstructures, in accordance with the invention; and

FIG. 39 illustrates an exemplary coaxial waveguide microstructure withsolder affixed thereto, in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The exemplary processes to be described involve a sequential build tocreate microstructures containing metal, dielectric and gas or a vacuousatmosphere. In the sequential build process, a structure is formed bysequentially layering various materials in a defined manner. Whenimplemented with lithographic patterning and other optional processes,for example, planarization techniques, a flexible method to form avariety of components, such as the exemplified suspended coaxialwaveguide microstructures, is provided.

The sequential build process is generally accomplished through processesincluding one or more of: (a) metal coating, sacrificial photoresistcoating, and dielectric coating processes; (b) surface planarization;(c) photolithography; and (d) etching or other layer removal processes.In depositing metal, plating techniques have been found to beparticularly useful, although other metal deposition techniques such asphysical vapor deposition (PVD) and chemical vapor deposition (CVD)techniques may be used. A typical sequential build process whichincludes a plating technique is described as follows.

An optionally patterned, conductive base or seed layer is formed at alllocations to be plated with metal. Patterning can be accomplished, forexample, by selective deposition of the conductive base or seed layer,such as by shadow masking, etching the seed layer using a resist andetchant, or by applying thin passivation to the seed layer such as athin permanent dielectric, such methods being known in the art ofplating on seed layers. A photoresist pattern, or mold, is formed overthe base or seed layer by coating and patterning. Next, a metalstructure is plated on all areas at which the base layer or seed layeris exposed until a desired thickness is achieved. The resultingstructure is optionally planarized to eliminate significant thicknessvariations from being carried into subsequent iterations of the processsequence.

One or more dielectric layers may be deposited at a desired point in theprocess sequence. For example, a dielectric layer may be formed prior toseed layer formation if metal plating over exposed dielectric materialin a subsequent plating step is desired, or after seed layer formationif plating over the exposed dielectric material is to be prevented. Thedielectric layer may, for example, be formed by spin coating andpatterning of the dielectric. This technique is useful, for example,where the patterned layer does not adversely affect a resist layerformed thereon, for example, where the thickness of the patterneddielectric (e.g., several microns) is significantly less than a resist(e.g., 100 μm) formed thereon. Another technique for forming thedielectric features involves pre-patterning the plating base or seedlayer in such a way that metal is not formed on certain regions of thebase layer after formation of the resist pattern/mold, but rather leavesempty pockets which can subsequently be filled with the dielectricmaterial. In this case, the dielectric material is typically filled tothe level of the resist mold or above the resist mold, and is planarizedwith the resist and any metal to create a planar layer before the nextprocess sequence begins.

After all of the structural layers have been created, the sacrificialresist is removed from the structure, leaving the other materialsincluding the dielectric in place. This may be accomplished by use of asolvent or developer in which the resist is soluble and the othermaterials are insoluble or relatively insoluble. Optional finishingsteps such as chemical polishing and/or overcoat plating may be desired.

The methods and devices of the invention will now be described withreference to FIGS. 2-13, which show cross-sectional and top-down viewsof an exemplary coaxial waveguide microstructure during various stagesof formation in accordance with one aspect of the invention. The coaxialwaveguides are useful for propagation of radio-frequency (RF) energy,for example, from several MHz to 100 GHz or more such as millimeterwaves and microwaves, as well as DC signals.

Referring to FIG. 2, the waveguide is formed on a substrate 2 which maybe, for example, a ceramic, a semiconductor, a metal, or a polymer. Thesubstrate can take the form, for example, of a printed wiring board or asemiconductor substrate, such as a silicon or gallium arsenide wafer.The substrate may have an expansion coefficient similar to the materialsused in forming the waveguide, and should be selected so as to maintainits integrity during formation of the waveguide. Also, the surface ofthe substrate 2 on which the waveguide is to be formed is typicallyplanar. The substrate surface may, for example, be lapped and/orpolished to achieve a high degree of planarity. Planarization of thesurface of the structure being formed can be performed before or afterformation of any of the layers during the process. Conventionalplanarization techniques, for example, chemical-mechanical-polishing(CMP), lapping, or a combination of these methods are typically used.Other known planarization techniques, for example, mechanical finishingsuch as mechanical machining, diamond turning, plasma etching, laserablation, and the like, may additionally or alternatively be used.

A base layer 4 is deposited over the substrate 2 and forms a bottom wallof the waveguide outer conductor in the final waveguide structure. Thebase layer 4 may be formed of a material having high conductivity, suchas a metal or metal-alloy (collectively referred to as “metal”), forexample copper, nickel, aluminum, chromium, gold, titanium, alloysthereof, a doped semiconductor material, or combinations thereof, forexample, multiple layers of such materials. The base layer 4 isdeposited by a conventional process, for example, by plating such aselectrolytic and/or electroless plating, physical vapor deposition (PVD)such as sputtering, or chemical vapor deposition (CVD). Plated copper isbelieved to be particularly suitable as the base layer material, withsuch techniques being well understood in the art. The plating can be,for example, an electroless process using a copper salt and a reducingagent. Suitable materials are commercially available and include, forexample, CIRCUPOSIT™ form electroless copper, available from Rohm andHaas Electronic Materials, L.L.C., Marlborough, Mass. Alternatively, thematerial can be plated by use of an electrically conductive seed layerfollowed by electrolytic plating. Suitable electrolytic materials arecommercially available and include, for example, COPPER GLEAM™ acidplating products, available from Rohm and Haas Electronic Materials,L.L.C. The use of an activated catalyst followed by electrolyticdeposition can alternatively be used. The base layer (and subsequentlayers) may be patterned into arbitrary geometries to realize a desireddevice structure through the methods outlined.

The thickness of the base layer (and the subsequently formed other wallsof the waveguide outer conductor) is selected to provide mechanicalstability to the waveguide and to provide sufficient conductivity forthe electrons moving through the waveguide. At very high frequencies,structural and thermal conductivity influences become more pronounced,as the skin depth will typically be less than 1 μm. The thickness thuswill depend, for example, on the specific base layer material, theparticular frequency to be propagated, and the intended application. Forexample, in instances in which the final structure is to be removed fromthe substrate, it may be beneficial to employ a relatively thick baselayer, for example, from about 20 to 150 μm or from 20 to 80 μm, forstructural integrity. Where the final structure is to remain intact withthe substrate, it may be desired to employ a relatively thin base layerwhich may be determined by the skin depth requirements of thefrequencies used. The base layer 4 may optionally next be planarized bythe techniques described above.

With reference to FIG. 3, a photoresist layer 6 is deposited over thebase layer 4, and is exposed and developed to form a pattern 8 forsubsequent deposition of lower sidewall portions of the waveguide outerconductor. The pattern 8 includes two parallel channels in the resist,exposing the top surface of the base layer 4. Conventionalphotolithography steps and materials can be used for this purpose. Theresist can be, for example, a positive or negative resist, such asShipley BPR™-100, PHOTOPOSIT™ SP or PHOTOPOSIT™ SN, available from Rohmand Haas Electronic Materials, L.L.C., or a dry film, such as theLAMINAR™ dry films, also available from Rohm and Haas ElectronicMaterials, L.L.C.

As shown in FIG. 4, lower sidewall portions 10 of the waveguide outerconductor are next formed. Appropriate materials and techniques forforming the sidewalls are the same as those mentioned above with respectto the base layer. The sidewalls are typically formed of the samematerial used in forming the base layer 4, although different materialsmay be employed. In the case of a plating process, the application of aseed layer or plating base may be omitted as here when metal in asubsequent step will only be applied directly over a previously formed,exposed metal region. It should be clear, however, that the exemplifiedstructures shown in the figures typically make up only a small area of aparticular device, and metallization of these and other structures maybe started on any layer in the process sequence, in which case seedlayers are typically used.

Surface planarization at this stage can be performed in order to removeany unwanted metal deposited on the top surface of the resist inaddition to providing a flat surface for subsequent processing. Throughsurface planarization, the total thickness of a given layer can becontrolled more tightly than might otherwise be achieved through coatingalone. For example, a CMP process can be used to planarize the metal andthe resist to the same level. This may be followed, for example, by alapping process, which slowly removes metal, resist, and any dielectricat the same rate, allowing for greater control of the final thickness ofthe layer.

As shown in FIG. 5, a support layer 12 of a dielectric material is nextdeposited over the resist layer 6 and the lower sidewall portions 10. Insubsequent processing, support structures are patterned from the supportlayer 12 to support the waveguide center conductor to be formed. Asthese support structures will lie in the core region of the finalwaveguide structure, the support layer 12 should be formed from amaterial such that the support structures do not create excessive lossesfor the energy to be transmitted through the waveguide. The materialshould also be capable of providing the mechanical strength necessary tosupport the center conductor and should be relatively insoluble in thesolvent used to remove the sacrificial resist from the final waveguidestructure. The support layer 12 material is typically a dielectricmaterial selected from: inorganic materials, such as silicas and siliconoxides, SOL gels, various glasses, silicon nitride (Si₃N₄), aluminumoxides such as alumina (Al₂O₃), aluminum nitride (AlN), and magnesiumoxide (MgO); organic materials such as polyethylene, polyester,polycarbonate, cellulose acetate, polypropylene, polyvinyl chloride,polyvinylidene chloride, polystyrene, polyamide, and polyimide;organic-inorganic hybrid materials such as organic silsesquioxanematerials; a photodefinable dielectric such as a negative actingphotoresist or photoepoxy which is not attacked by the sacrificialresist removal process to be conducted. It is advantageous to usematerials which can be easily deposited, for example, by spin-coating,roller coating, squeegee coating, spray coating, chemical vapordeposition (CVD), or lamination. The support layer 12 is deposited to athickness that provides for the requisite support of the centerconductor without cracking or breakage. In addition, the thicknessshould not severely impact subsequent application of resist layers fromthe standpoint of planarity.

Referring to FIG. 6, the support layer is next patterned using standardphotolithography and etching techniques to provide a plurality ofconductor supports 12′. Plural conductor supports 12′ may be associatedwith both of the lower sidewall portions 10. One end of each of thesupports 12′ is formed over one or the other lower sidewall portion 10and the opposite end extends to a position over the resist layer 6between the sidewall portions 10. The supports 12′ on each sidewallportion are spaced apart from one another, typically at a fixeddistance, and those on one sidewall portion are typically offset fromthose on the opposite sidewall portion so as not to be directly oppositeone another. In an alternative embodiment, the support members 12′ canbe formed to extend from one sidewall portion 10 to the other. Thenumber, shape, and pattern of arrangement of the supports 12′ should besufficient to provide support to the center conductor while preventingexcessive signal loss. In addition, the shape and periodicity oraperiodicity may be selected to prevent reflections at frequencies wherelow loss propagation is desired, as can be calculated using methods knowin the art of creating Bragg gratings and filters, unless such functionis desired.

With reference to FIG. 7, a resist layer 6′ is deposited over thesubstrate, and is exposed and developed to form patterns 14 and 16 forsubsequent deposition of middle sidewall portions of the waveguide outerconductor and the center conductor, respectively. The pattern 14 for themiddle sidewall portion includes two channels coextensive with the twolower sidewall portions 10. The lower sidewall portions 10 and the endof the conductor supports 12′ overlying the lower sidewall portions areexposed by pattern 14. The pattern 16 for the center conductor is achannel parallel to and between the two middle sidewall patterns,exposing the opposite ends of and supporting portions of the conductorsupports 12′. Conventional photolithography steps and materials, such asthose described above, can be used for this purpose.

As shown in FIG. 8, the middle sidewall portions 18 of the outerconductor and the center conductor 20 are next formed by depositing asuitable material into the channels formed by the resist 6′. Appropriatematerials and techniques for forming the middle sidewall portions 18 andcenter conductor 20 are the same as those mentioned above with respectto the base layer 4 and lower sidewall portions 10, although differentmaterials and/or techniques may be employed. Surface planarization mayoptionally be performed at this stage to remove any unwanted metaldeposited on the top surface of the resist in addition to providing aflat surface for subsequent processing, as has been previously describedand optionally applied to any or every layer.

With reference to FIG. 9, a resist layer 6″ is deposited over thesubstrate, and is exposed and developed to form pattern 22 forsubsequent deposition of upper sidewall portions of the outer conductor.The pattern 22 for the middle sidewall portion includes two channelscoextensive with and exposing the two middle sidewall portions 18.Conventional photolithography steps and materials as described above canbe used for this purpose.

As illustrated in FIG. 10, upper sidewall portions 24 of the outerconductor are next formed by depositing a suitable material into thechannels formed by the resist 6″. Appropriate materials and techniquesfor forming the upper sidewalls are the same as those mentioned abovewith respect to the base layer and other sidewall portions. The uppersidewalls 24 are typically formed with the same materials and techniquesused in forming the base layer and other sidewalls, although differentmaterials and/or techniques may be employed. Surface planarization canoptionally be performed at this stage to remove any unwanted metaldeposited on the top surface of the resist in addition to providing aflat surface for subsequent processing.

With reference to FIG. 11, a resist layer 6′″ is deposited over thesubstrate, and is exposed and developed to form pattern 26 forsubsequent deposition of the top wall of the waveguide outer conductor.The pattern 26 for the top wall exposes the upper sidewalls 24 and thearea therebetween. Optionally, patterning of resist layer 6′″ will alsoresult in one or more regions of the resist layer 6′″ in the areabetween upper sidewall portions 24. These remaining portions can, forexample, be in the form of resist pillars 31. Subsequent deposition ofthe top wall of the outer conductor would be prevented in these regionsof remaining resist 6′″. As described in greater detail below, thisresults in openings in the subsequently deposited outer conductor topwall facilitating removal of resist from the final structure.

As shown in FIG. 12, the top wall 28 of the outer conductor is nextformed by depositing a suitable material into the exposed region overand between the upper sidewall portion 24. As can be seen, deposition isprevented in the volume occupied by and on the resist pillars 31.Appropriate materials and techniques for forming the upper sidewalls arethe same as those mentioned above with respect to the base layer andsidewall portions. The top wall 28 is typically formed with the samematerials and techniques used in forming the base layer and othersidewalls, although different materials and/or techniques may beemployed. Surface planarization can optionally be performed at thisstage.

With the basic structure of the waveguide being complete, additionallayer may be added or the resist remaining in the structure may next beremoved. The resist may be removed by known solvents or removers basedon the type of resist used. In order for the resist to be removed fromthe structure, the solvent must come into contact with the resist. Theresist is exposed at the end faces of the waveguide structure.Additional openings in the waveguide such as described above withrespect to FIGS. 11-12 may be provided to facilitate contact between thesolvent and resist throughout the structure. Other structures forallowing contact between the resist and solvent are envisioned. Forexample, openings can be formed in the waveguide sidewalls during thepatterning process. The dimensions of these openings may be selected tominimize interference with, scattering or leakage of the guided wave.The dimensions can, for example, be selected to be less than ⅛ or 1/10of the wavelength of the highest frequency used.

The final waveguide structure 32 after removal of the sacrificial resistis shown in FIG. 13. The space previously occupied by the sacrificialresist within the outer walls of the waveguide forms the waveguide core30. This volume is typically occupied by air. However, it is alsoenvisioned that a gas having better dielectric properties than air maybe used in the core or a vacuum can be created in the core, for example,when the structure forms part of a hermetic package. As a result, areduction in absorption from water vapor that would otherwise adsorb tothe surfaces of the waveguides can be realized.

It should be noted that the coaxial waveguide microstructures describedabove are exemplary, and other structures are envisioned. For example,FIGS. 14-26 illustrate a further exemplary waveguide structure whichemploys a different center conductor support structure. The descriptionabove with respect to FIGS. 2-13 is generally applicable to thisembodiment with differences being outlined below.

As shown in FIG. 15, the conductor support members are formed bydepositing a passivation layer 9 over the base layer 4. The passivationlayer is a material upon which the conductive material making up thewaveguide walls and conductor does not deposit. Suitable materials forthe passivation layer include, for example, a photodefinable dielectricsuch as a negative acting photoresist or photoepoxy which is notattacked by the sacrificial resist removal process to be conducted. Thepassivation layer may alternatively be also be a thinner version of thestructural dielectric that is used to fill the gaps in subsequent step.The passivation layer can be formed by known methods such asspin-coating, roller coating, or vapor deposition.

The passivation layer 9 is next patterned using standardphotolithography (for photodefinable composition) or photolithographyand etching techniques to form passivation layer 9′ on which the centerconductor supports are to be formed, as illustrated in FIG. 16.

With reference to FIG. 17, a photoresist layer 6 is deposited over thebase layer 4 and passivation layer 9′, and is exposed and developed toform parallel channels 8 for subsequent deposition of lower sidewallportions of the waveguide as described above, and windows 11 over thepassivation layer 9′ where the conductor supports are to be formed. Asshown in FIG. 18, the lower sidewall portions 10 of the waveguide arenext formed as described above.

A dielectric material is next deposited over the substrate surface. Thedielectric material may be any of those described above with referenceto the support structures of the first exemplary waveguidemicrostructure. The material can be blanket-deposited across the entiresurface of the substrate followed by a planarization, for example, byCMP, such that the dielectric material 12′ fills the windows over theexposed passivation material 9′ and is limited to those areas, asillustrated in FIG. 19. The dielectric material can alternatively beselectively deposited in those areas by known techniques, for example,by spin coating, screen printing, resist plating, or vapor deposition.

With reference to FIG. 20, after deposition of a seed layer as a platingbase, a resist layer 6′ is deposited over the substrate, and is exposedand developed to form patterns 14 and 16 for subsequent deposition ofmiddle sidewall portions of the waveguide and the center conductor,respectively, as described above.

The middle sidewall portions 18 and center conductor 20 of the waveguideare next formed by depositing a suitable material into the channelsformed by the resist, as illustrated in FIG. 21. As shown, the centerconductor 20 is formed on the conductor supports 12′. As shown in FIGS.22-26, the waveguide structure is completed as described above withrespect to the previous exemplary embodiment.

For certain applications, it may be beneficial to remove the finalwaveguide structure from the substrate to which it is attached. Thiswould allow for coupling on both sides of the released interconnectnetwork to another substrate, for example, a gallium arsenide die suchas a monolithic microwave integrated circuit or other devices. Releaseof the structure from the substrate may be accomplished by varioustechniques, for example, by use of a sacrificial layer between thesubstrate and the base layer which can be removed upon completion of thestructure in a suitable solvent. Suitable materials for the sacrificiallayer include, for example, photoresists, high temperature waxes, andvarious salts.

FIGS. 27 and 28, respectively, illustrate cross-sectional views of aplurality of the first and second exemplary coaxial waveguidemicrostructures in a stacked arrangement 34. The stacked arrangement canbe achieved by continuation of the sequential build process through eachstack, or by preforming the waveguides on individual substrates,separating waveguide structures from their respective substrates using arelease layer, and stacking the structures. In theory, there is not alimit on the number of waveguides that can be stacked using the processsteps discussed herein. In practice, however, the number of layers willbe limited by the ability to manage the thicknesses and stresses andresist removal associated with each additional layer. The spaceutilization of the wave-guides can be very high since adjacentwaveguides can share common sidewalls. While the waveguides illustratedare parallel, alternative designs can include, for example, any planargeometry, for example, those resulting in splitters, combiners,circulators, branching networks, and the like. Consequently, thewaveguides can be angled with respect to each other and producecurvature in their plane to reduce losses.

FIGS. 29-31 illustrate exemplary structures which allow waveguide coreson different levels to be joined together using vias. In these exemplaryembodiments, a surface or part of a wall region of the outer conductorof various waveguides are patterned to expose an opening between twoadjacent waveguides. The waveguide cores can be connected using viascreated contemporaneously with the sidewalls and top surfaces. In theseexamples, the vias shown in section AA can be fabricated in three ormore plating steps, excluding the waveguide core layers. Other methodsof coupling between layers may be used, for example, by making multiplesteps or stairs or making a coupler as described with reference to FIG.32.

FIG. 30 illustrates a terminating stub creating a transition to a hollowwaveguide that ends in an opening or aperture to free space or anotherdevice exterior to the waveguide network. Such a stub can be designed toallow efficient conversion, for example, between a square hollowwaveguide and coaxial propagating modes. This structure forms the basisfor creating an antenna or radiator.

FIG. 31 shows a tapered structure that can be made in one or more stepsor by using grey scale resist processing or mechanical machining. Suchtapered structures can be used to match impedances in a hollowwaveguide, and also to concentrate an incoming signal or wave from freespace to the stub.

In accordance with a further exemplary aspect of the invention, thestructures may be comprised of layers of structural photosensitivedielectric polymers such as resists that are metallized on theirsurfaces. In this case, the thickness of the metallization can bedetermined by the skin depth requirements for the metals chosen at thefrequencies of operation and may, for example be from about 0.2 to 3 μmfor typical applications. Additional thickness and other metallicstructures such as posts may be included for other reasons such aspiping heat from any integrated active devices.

The waveguides of the invention typically are square in cross-section.Other shapes, however, are envisioned. For example, other rectangularwaveguides can be obtained in the same manner the square waveguides areformed, except making the width and height of the waveguides different.Rounded waveguides, for example, circular or partially roundedwaveguides can be formed by use of gray-scale patterning. Such roundedwaveguides can, for example, be created through conventional lithographyfor vertical transitions and might be used to more readily interfacewith external micro-coaxial conductors, to make connector interfaces,etc.

FIG. 32 illustrates two parallel waveguides cores 40, 42 which arebrought into close proximity to each other for a short distance suchthat the common outer conductor sidewall 44 between them gradually butmomentarily disappears over a predetermined distance and then reappears.This structure creates a gap 46 in the sidewall. Removal of the sidewallportion over the pre-determined distance allows for controlled couplingbetween the waveguides. Within gap 46, cross talk will occur between theRF signal on one core and the neighboring core. This geometry allowscomponents such as RF splitters and attenuators to be created and isapplicable to waveguides with and without center conductors. With such astructure, the desired amount of coupling can more accurately becontrolled resulting in greater ease to achieve a desired split ratio.Well-defined splitters and couplers are highly desired to createsophisticated RF and microwave networks. More than one controlledperforation in the wall between the waveguides can optionally be usedfor the same purpose. Techniques such as offsetting the centerconductor, changing the exterior waveguide dimensions in the region ofthe coupler, and other structural modifications may be utilized tooptimize the desired effect.

For certain applications, it may be beneficial to include one or morecoaxial-type and hollow-type waveguides on the same substrate. FIGS.33A-33B illustrate exemplary transition structures for connecting acoaxial waveguide 48 to a hollow transmission-type waveguide 50. Suchtransition can be accomplished through means known in the art such ascreating a center conductor probe, wherein one or more of thecoaxial-type waveguides terminates with short stubs into the hollow-typewaveguide structures. These transitions can occur between and within thelayers. The distance d between the walls and the stub as may be adjustedto maximize the efficiency of these radiators.

Depending on the specific materials used for the waveguide structure andthe substrate to be coupled with the structure, it may be beneficial toprovide a flexible or compliant interface between the two to compensatefor coefficient of thermal expansion (CTE) mismatch. This can take theform, for example, of flexible fingers or posts perpendicular to thesubstrate or interface with narrow gaps between the fingers or postsallowing flexibility and compliance as needed. Other techniques include,for example, compliant and conductive bumps, springs, rings connected byposts, or flex circuitry. FIG. 34 illustrates a compliant interface inthe form of a spring structure 52, connecting a waveguide 32 to asubstrate 2.

Using such a technique, the first layer built on a substrate can be, forexample, a series of center conductors surrounded by circular orrectangular springs spaced with less than one quarter wave between therings and connected with one or more spaced posts between the rings.Also, rings may be made connected by short posts separated by a distancetypically less than a quarter, for example, less than 1/10 of thewavelength at the highest frequency desired allowing spring likevertical connections for both compliance in connectors and to help dealwith coefficient of thermal expansion (CTE) mismatch between theselayers and other materials such as silicon and gallium arsenide.

Optionally, the walls of the waveguide can be formed such that they arenot continuous. The walls can, for example, be made of interconnectedvias, posts, spirals, or springs with the elements being spaced apart soas to minimize or prevent loss at the desired frequency. Typically, thisdistance is less than ¼ of a wavelength at the desired frequency ofoperation, for example, less than 1/10 of such wavelength. Furtheroptionally, a combination of the above together with continuous wallsmay be used. For example the top and bottom walls may be planar and thevertical surfaces composed of interconnecting vias. Such wall structurescan optionally be utilized throughout the interconnect substrate.Additional benefits of non-continuous wall structures include one ormore of the ability to make compliant structures that can better dealwith CTE mismatch of integrated die or substrates, better ability toremove the resist or obtain fluid flow in and from the substrate, bettertuned operating characteristics, and compliance in interconnections andconnectors.

With reference to FIG. 35, a dielectric cap layer, or membrane, 54 canoptionally be formed over the end faces of the waveguide microstructureto protect the waveguide from dust and other contamination or to serveas a dielectric platform on the top or bottom surfaces to hybridly mountdevices. In the exemplified structure, the cap layer 54 covers the outerperiphery of the center conductor end surface and the inner periphery ofthe outer conductor end surface, to allow electrical and thermalcontinuity, while also protecting the waveguide from contamination. Thecap layer can be formed, for example, by depositing the material, forexample, by spin-coating and patterning a photoimageable dielectriclayer that remains. The material can be, for example, a photopatternablepolymer that is relatively insoluble in the medium used to remove theresist from the final structure, for example, a photoresist. Vent holes(not shown) may be provided in the cap layer to prevent stress thereinand to facilitate dissolution and removal of the resist in the corevolume.

It may be desired to connect multiple waveguide structures together orto connect the waveguide structures to other structures, for example,when mating a wafer or device, for example, an RF device such as amicrowave integrated circuit device, a microwave component, or othersurface mount component, as a temporary connection when testing thewaveguide structure, or when replacing parts. A number of interlockinggeometries can be utilized to interconnect the waveguides with otherwaveguides or other components. For example, overlapping tubes, tubesand pins, slots and keys for the outer conductor and inner conductor, ifpresent, may be used for this purpose. FIGS. 36-38 illustrate threeexemplary interlocking connection structures. FIGS. 36-37 illustrate twoslot/key connection structures. FIG. 38 shows an overlapping tubeconnection structure in which the waveguide 56 has a flared section 58at an end portion thereof, such that that the flared section may be slidover an end portion of waveguide 60. The connection can be friction-fit,soldered and/or may be held in place with other adhesive material. Thesestructures can be formed during fabrication of the waveguides usingstandard photolithography techniques.

As shown in FIG. 39, a thin layer of solder 62 may be deposited by knownmeans, for example, by electroplating or evaporation to facilitatebonding passive and/or active devices 64 to the waveguides, or forbonding the waveguide structure to a different substrate. The solder canbe deposited on any vertical or horizontal exposed face and before orafter removal of the photoresist from the waveguide core volume.

One may also desire to coat the interior walls of the waveguide outerconductor and/or the center conductor with a metal chosen for low lossat the frequencies used, such as gold, nickel, copper, nickel-iron,palladium, platinum, or gold or a combination of these metals, such asnickel and gold. This can be accomplished, for example, by a platingprocess after removal of the sacrificial resist from the waveguidestructure.

For certain applications, it may be desired to form electricalfeedthroughs in the substrate or otherwise form holes or structures inthe substrate that aid in transmission and reception of propagatingwaves, similar to the structure shown in FIG. 31. These features can beformed in the substrate, for example, by machining or other knownpatterning techniques. For example, it is envisioned that horn antennascan be made part of the substrate, for example, as shown in FIG. 25 byanisotropically etching a pit followed by metallization. Themetallization step can optionally be conducted together with other stepsin the sequential build process.

While the invention has been described in detail with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made, and equivalentsemployed, without departing from the scope of the claims.

What is claimed is:
 1. A coaxial waveguide microstructure, comprising:at least one coaxial waveguide, comprising: a center conductor; an outerconductor comprising one or more walls, spaced apart from and disposedaround the center conductor; one or more dielectric support members forsupporting the center conductor in contact with the center conductor andenclosed within the outer conductor; and a core volume between thecenter conductor and the outer conductor, wherein the core volume isunder vacuum or in a gas state and wherein the center conductor and theouter conductor of the at least one coaxial waveguide each having agenerally rectangular cross-sectional structure.
 2. The coaxialwaveguide microstructure of claim 1, wherein the outer conductorcomprises a first sidewall, and a plurality of the one or moredielectric support members have a first end portion embedded in thefirst sidewall and extend beneath the center conductor.
 3. The coaxialwaveguide microstructure of claim 2, wherein the outer conductorcomprises a second sidewall opposite the first sidewall, and a secondplurality of the one or more dielectric support members have a first endportion embedded in the second sidewall and extend beneath the centerconductor.
 4. The coaxial waveguide microstructure of claim 1, whereinthe one or more dielectric support members are in the form of supportposts having a first end on an inner surface of the outer conductor anda second end in contact with the center conductor.
 5. A hermeticpackage, comprising the coaxial waveguide microstructure of claim
 1. 6.The coaxial waveguide microstructure of claim 1, wherein the at leastone coaxial waveguide comprises a plurality of coaxial waveguides,wherein the plurality of coaxial waveguides are present in a stackedarrangement.
 7. The coaxial waveguide microstructure of claim 6, whereinthe plurality of coaxial waveguides are joined together by one or morevias.
 8. The coaxial waveguide microstructure of claim 1, comprising aplurality of the one or more dielectric support members.
 9. The coaxialwaveguide microstructure of claim 1, wherein the center conductor andthe outer conductor are formed of copper or a copper-alloy and comprisea coating including gold, copper, palladium, nickel-iron, platinum,nickel, or combinations thereof.
 10. The coaxial waveguidemicrostructure of claim 1, comprising a substrate over which the atleast one coaxial waveguide is disposed and comprising a sacrificialrelease layer between the substrate and the waveguide.
 11. The coaxialat least one coaxial waveguide microstructure of claim 1, wherein the atleast one coaxial waveguide further comprises a connectorizationstructure at an end portion of the at least one coaxial waveguidecomprising a slot structure, a key structure or a flared structure. 12.The coaxial at least one coaxial waveguide microstructure of claim 1,wherein the at least one coaxial waveguide further comprises adielectric membrane over an end face of the at least one coaxialwaveguide extending from the center conductor to the outer conductor.13. The coaxial waveguide microstructure of claim 1, comprising asubstrate over which the at least one coaxial waveguide is disposed andcomprising a compliant interface structure between the substrate and atleast one coaxial waveguide to compensate for CTE mismatch therebetween.14. The coaxial waveguide microstructure of claim 1, wherein the atleast one coaxial waveguide comprises a plurality of the one or moredielectric support members; the center conductor and outer conductorcomprise copper or a copper alloy; and further comprising an activedevice bonded to the waveguide.
 15. A method of forming a coaxialwaveguide microstructure by a sequential build process, comprising: (a)depositing a plurality of layers over a substrate, wherein the layerscomprise one or more of a metal material, a sacrificial photoresistmaterial, and a dielectric material, thereby forming a structure abovethe substrate, comprising: a center conductor; an outer conductorcomprising one or more walls, spaced apart from and disposed around thecenter conductor; one or more dielectric support members for supportingthe center conductor, in contact with the center conductor and enclosedwithin the outer conductor; and a core volume between the centerconductor and the outer conductor, wherein the core volume comprisessacrificial photoresist; and (b) planarizing one or more of the layersof (a) by one or more of plasma etching, diamond turning, and laserablation; (c) removing the sacrificial photoresist from the core volume.16. The method of claim 15, further comprising after (c), (d) evacuatingthe core volume to a vacuous state.
 17. The method of claim 15, whereinthe outer conductor comprises a first sidewall, and a plurality of thedielectric support members have a first end portion embedded in thefirst sidewall and extend beneath the center conductor.
 18. The methodof claim 15, further comprising after (c), (d) separating the substratefrom the structure.